1. Field of the Invention
The present invention relates to an acceleration sensor, which in particular is capable of detecting accelerations acting upon three axes, respectively.
2. Background Information
In recent years, acceleration sensors are widely used in all kinds of precision instruments, cars, robots and other various industrial fields. Especially, there is an increasing demand for a semiconductor acceleration sensor, which uses an MEMS (micro electro mechanical system) technology, as it is small in size, light, accurate and reliable in operation, and available at low cost.
In most semiconductor acceleration sensors, the piezoresistance effect, i.e. a phenomenon in which an electric resistivity changes in proportion to stress, is used in detecting acceleration. A common semiconductor acceleration sensor, for instance, is formed by having a semiconductor acceleration sensor chip fixed on a pedestal of a ceramic substrate forming a portion of a package. This semiconductor acceleration sensor chip has a plummet disposed in a central portion of the semiconductor acceleration sensor chip and four flexible beams which hang the plummet such that the beams will bend in response to the accelerations acting on the plummets. The four flexible beams and the plummet are formed by processing one semiconductor substrate. On the surfaces of the four flexible beams, piezo elements are formed, respectively, and these piezo elements constitute a Wheatstone bridge circuit. When stress occurs at the beam by the act of acceleration, the resistance balance of the Wheatstone bridge will change, and the acceleration is detected by having this resistance change measured as a current change or a voltage change.
For example, inventions relating to the acceleration sensor are disclosed in Japanese Patent No. 2127840, p. 11, FIG. 11 (hereinafter to be referred to as Patent Reference 1), Japanese Laid Open Patent Application No. 9-61448, pp. 2-3, FIG. 3 (hereinafter to be referred to as Patent Reference 2) and Japanese Laid Open Patent Application No. 10-123166, pp. 5-8, FIGS. 1-6 (hereinafter to be referred to as Patent Reference 3).
The invention disclosed in Patent Reference 1 relates to a semiconductor acceleration sensor using piezo elements, and it has an acting unit (a plummet portion) in a peripheral portion of a silicon monocrystal substrate that performs the main functions of the semiconductor acceleration sensor, flexible portions (beam portions) inside the acting unit, and a fixed portion in a central portion of this silicon monocrystal substrate. The monocrystal substrate is fixed directly to the bottom face of the package through the conical pedestal disposed on the under surface of the fixed portion. When a cylindrical plumb disposed on the under surface of the acting portion (plummet portion) is displaced due to acceleration, this displacement is transmitted to the flexible portions (beam portions) of the monocrystal substrate, and stress accrues on piezo elements formed on the flexible portions. Thereby, the acceleration applied to the semiconductor acceleration sensor is detected. With this semiconductor acceleration sensor structure, the displacement of the plumb in the right and left directions is made to stay within a predetermined limit by a gap formed between the plumb and the pedestal.
The invention disclosed in Patent Reference 2 relates to an acceleration sensor using piezoelectric ceramics. With respect to the invention disclosed in Patent Reference 2, in mounting the acceleration sensor of which one end is closed on a circuit substrate, an integrated circuit for sensor driving is disposed in between the acceleration sensor and the circuit substrate. Due to such arrangement, the mounting area for the acceleration sensor can be minimized, the circuit pattern of the circuit substrate can be shortened, and noise resistance can be improved.
The invention disclosed in Patent Reference 3 relates to a semiconductor acceleration sensor using a piezo element. With respect to the invention disclosed in Patent Reference 3, a semiconductor acceleration sensor chip having a plummet portion hung by beam portions is fixed on a pedestal which is formed by a material having the same thermal expansion coefficient as the semiconductor acceleration sensor, and the pedestal and the plummet portion are disposed closely such that an air gap between the two stays within a range of 7 to 15 μm. With this semiconductor acceleration sensor structure, it is possible to damp the vibrations of the plummet portion by means of air damping between the plummet portion and the pedestal, and stabilize the output characteristic of the sensor, by which a comparatively low level of acceleration can be detected.
In order to minimize the size of the semiconductor acceleration sensor, it is necessary to minimize and reduce the thickness of the plummet portion. Normally, when the plummet portion is minimized or made thinner, the moment of inertia becomes smaller, which desensitizes the acceleration sensor. Therefore, in this case, it is also necessary to reduce the thickness of the flexible beam portion in order to raise the sensitivity of the acceleration sensor. However, making the beam portion thinner deteriorates the shock-resistance of the semiconductor acceleration sensor, and the semiconductor acceleration sensor may become vulnerable to external shocks, such as shocks caused by dropping. Moreover, when the beam portion is made thinner, the semiconductor acceleration sensor may be damaged by receiving shocks in its manufacturing process, which can result in a reduction in the yield ratio. For instance, the semiconductor acceleration sensor may be damaged by water pressure during the dicing process or by force that can be applied to it during pick up. Considering these problems, a semiconductor acceleration sensor which can have a small size and thin configuration, and which can have improved shock-resistance, is required.
In the acceleration sensor of Patent Reference 1, the fixed portion is formed in the central portion of the silicon monocrystal substrate, and the monocrystal substrate is fixed directly to the bottom face of the package through the conical pedestal disposed on the under surface of the fixed portion. Therefore, it is necessary to have a process of forming a gap between the under surface level of the cylindrical plumb disposed on the under surface of the acting unit (plummet portion) in the peripheral portion, and the under surface level of the conical pedestal disposed on the under surface of the fixed portion. It is a problem because such process may complicate the overall manufacturing process. In addition, Patent Reference 1 does not make any reference to the shock-resistance of the acceleration sensor, especially the shock-resistance of the beam portion.
In the acceleration sensor of Patent Reference 2, a plummet portion is disposed in a central portion of the bottom-bearing cylindrical acceleration sensor to which piezoelectric ceramics are used as its material. Therefore, in order to minimize the size of the acceleration sensor, it is necessary to minimize and reduce the thickness of the plummet portion and to reduce the thickness of the flexible portions having sensor functions at the same time. Accordingly, it is a problem because the shock-resistance of the acceleration sensor may be deteriorated due to such arrangement, and the acceleration sensor may become vulnerable to external shocks.
In the acceleration sensor of Patent Reference 3, the plummet portion is formed in a central portion of a semiconductor substrate, and it is supported by a frame body in a peripheral portion through the flexible beam portion. Therefore, in order to minimize the size of the acceleration sensor, it is necessary to minimize and reduce the thickness of the plummet portion and to reduce the thickness of the flexible beam portion at the same time. Accordingly, it is a problem because the shock-resistance of the acceleration sensor may be deteriorated due to such arrangement, and the acceleration sensor may become vulnerable to external shocks.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved acceleration sensor. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.